One type of plasma device includes a vacuum chamber having a metal wall and base at ground, i.e. a reference, potential. A plasma is formed in the vacuum chamber in response to AC excitation, typically from an r.f. or microwave source. The r.f. excitation is, in some types of devices, derived from a coil having turns extending radially and circumferentially, such as a circular or "rectangular" spiral. Magnetic and electric r.f. fields from the coil are coupled to ionizable material (usually a gas) in the vacuum chamber to form the plasma. The plasma is incident on a workpiece, such as a circular semiconductor wafer or a rectangular, glass sheet used in a flat panel display. The plasma etches material from or deposits material onto the workpiece. The periphery of the coil has a size and shape generally corresponding with the workpiece peripheral size and shape. Typically, the coil is located outside the vacuum chamber to derive r.f. magnetic and electric fields coupled to the ionizable material in the vacuum chamber through a dielectric window of the vacuum chamber. In many instances, the workpiece is mounted on a metal chuck biased by an r.f. source to attract charge particles in the plasma to the workpiece.
Other systems have been proposed in which a coil is immersed in the vacuum chamber, so that the plasma surrounds at least part of the coil. Such immersed coils are resistively coupled to the plasma since the main impedance component of the plasma is resistive and the plasma contacts the coil. In contrast, coils located outside the chamber and coupled to the plasma through the dielectric window are reactively coupled to the plasma by the electric and magnetic fields. Locating the coil outside of the vacuum chamber results in more efficient coupling of magnetic fields to the plasma than is achieved by an immersed coil. The immersed coil experiences greater power losses than the external coil because of capacitive coupling between the immersed coil and the plasma. This is because the plasma excited by the immersed coil has a lower flux density than the plasma established by the external coil. The high flux density plasma which is established by the external coil leads directly to high deposition and etching rates of plasma materials on the workpiece.
In the past, materials have been deposited on workpieces in vacuum plasma processing chambers by chemical vapor deposition (CVD) processes carried out in the presence of r.f. and microwave excited plasmas. Molecules containing atoms desirably deposited on the workpiece are introduced into the vacuum chamber and chemically reacted with the assistance of the plasma to dissociate the desired atoms from the remainder of the molecules. The desired atoms are frequently in complicated organic molecules including many atoms other than the atoms desirably deposited on the workpiece. Many of the atoms in the molecules, other than those desirably deposited on the workpiece, are frequently deposited on the workpiece, whereby the workpiece has a tendency to be contaminated with such atoms.
Plasmas excited by magnetic fields resulting from r.f. coil excitation are prone to instability. The magnetic fields typically operate on relatively low energy electrons. Relatively small fluctuations of the density of these low energy electrons and/or the temperature of the electrons affects coupling of the magnetic fields to the electrons, resulting in relatively large amplitude plasma impedance variations. These relatively large plasma impedance fluctuations are coupled to the coil and circuitry driving the coil, including a matching network connected between the coil and r.f. source. The plasma impedance fluctuations can be so severe as to cause plasma extinction. In any event, the instability of plasmas excited predominantly by magnetic fields, i.e., fields inductively coupled from a coil to the plasma, is a problem that has hampered operation of some coil-excited r.f. plasmas.
Inductively excited r.f. plasmas responsive to fields derived from coils are frequently difficult to ignite. To ignite ionizable gas in the vacuum chamber into a plasma discharge, relatively high voltages often must be applied to the coil to produce sufficiently high electric fields that are coupled from the coil to the plasma. The load seen by the coil prior to the plasma discharge being established is essentially capacitive but becomes primarily resistive when plasma ignition occurs. The sudden change in the impedance seen by the coil requires substantial changes in the matching network connected between the coil and the r.f. excitation source. Hence, it is desirable to provide some way of reducing the voltage necessary to achieve plasma ignition.
Dielectric windows of coil-driven r.f. plasma processors have a tendency to be clouded by material from the plasma being incident on the insides of the windows. The materials can be polymers from organic molecules etched from the workpiece or metal particles dissociated from molecules involved in chemical vapor deposition and other processes. Formation of the polymer on the dielectric window is undesirable since the deposition of the polymers on the window can lead to formation of particulates that lead to workpiece contamination. Clouding of the window by the metal adversely affects coupling of the r.f. fields from the coil external to the processor chamber through the window to the plasma. Typically, the prior art has dealt with this problem by opening the vacuum chamber and cleaning the window and the rest of the chamber interior or by using in situ methods which are performed under vacuum conditions, at times when processing operations are not performed. Consequently, substantial processor down time occurs as a result of the cleaning activities.
It is, accordingly, an object of the present invention to provide a new and improved vacuum plasma device.
Another object of the invention is to provide a new and improved apparatus for and method of depositing a non-magnetic metal on a workpiece in a vacuum plasma processing chamber.
A further object of the invention is to provide a new and improved apparatus for and method of depositing a non-magnetic metal on a workpiece in an r.f. plasma vacuum plasma processor wherein the deposit is substantially devoid of impurities and consists substantially only of the metal.
An additional object of the invention is to provide a new and improved apparatus for and method of igniting an ionizable gas to an a.c. plasma.
Still an additional object of the invention is to provide a new and improved apparatus for and method of stabilizing an r.f. coil-excited plasma in a vacuum chamber.
Yet a further object of the invention is to provide a new and improved apparatus for and method of cleaning a dielectric window of a vacuum processing chamber having an r.f. coil-excited plasma.
An added object of the invention is to provide a new and improved apparatus for and method of cleaning a dielectric window of a vacuum processing chamber having an r.f. coil-excited plasma wherein the window is cleaned substantially at the same time as plasma processing of a workpiece in the chamber.
Still another object of the invention is to provide a new and improved apparatus for and method of removing polymers etched from a processed workpiece from a dielectric window of a vacuum plasma processing chamber.
Still yet another object of the invention is to provide a new and improved vacuum plasma processor wherein non-magnetic metal deposited on a workpiece in the processor is removed from a dielectric window of the processor at substantially the same time as deposition of the metal on the workpiece, and to a method of obtaining such results.